Emissions of air pollutants have been increasing significantly in Asian countries due to the rapid development of industry and economy. Long-range, transboundary transport of these pollutants probably affects the atmospheric environment and the regional climate in this region (Kita et al. 2009). Climate change, air pollution, and sustainable development are inter-linked, and co-benefits of cutting short-lived climate pollutants (SLCP) will avoid global warming higher than 1.5 °C and negative trade-offs (CCAC 2019; IPCC 2018). Therefore, identification of SLCP emission/production/ transportation sources is critical for planning mitigative measures to reduce SLCP.

In this study, simultaneous observation of black carbon (BC), tropospheric ozone (TO3) and particulate matter 2.5 (PM2.5), which are significant climate forcers, was carried out at VNU Vietnam Japan University in Hanoi to clarify the concentrations and variations of SLCP in Hanoi and Northern Vietnam. The research applied HYSPLIT trajectory model to distinguish contribution source regions of SLCPs to Hanoi and deployed remote PM2.5 stations surrounding Hanoi and coastal region in Northeast sector of Northern Vietnam to compare upwind and downwind concentrations.

Figure C2.1 shows the maximum episodes of BC and PM2.5 observed in wintertime, especially in January with periods lasting from 1 day to 1 week. Monthly averaged concentrations of BC, TO3 and PM2.5 were in range of 1–3 μg/m3, 20–50 ppbv and 18–65 μg/m3, respectively (Fig. C2.2). BC concentration was estimated from 4% to 6% of PM2.5 in all seasons of 2019. Diurnal variations of these species suggested that major part of them were emitted or produced in Northern Vietnam region. BC and PM2.5 were remarkably increased during rush hours or night-time in diurnal variation. In contrast, TO3 was often high at noon and depleted to zero at night. Seasonal variation as shown in Fig. C2.2 indicated that BC and PM2.5 increased with winter monsoon, and TO3 actively produced in summer, indicating that air transport in association with the winter monsoon affected concentration of BC and PM2.5 in this region. Due to observed enhances of BC and PM2.5 in 2019, the comparison analysis with local and regional transport features focused on wintertime (Fig. C2.3). These high rises were mostly associated with trajectories from South China Sea, and detailed analyses of relation between these rises and the calculated trajectory routes revealed that these rises were actually attributed to emissions from North East coastal region of Northern Vietnam.

Fig. C2.1
figure 1

Timeseries of BC, TO3 and PM2.5 in Hanoi in Jan. and Feb., 2019

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Fig. C2.2
figure 2

Monthly average of BC, PM2.5 and TO3 in Hanoi in 2019

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Fig. C2.3
figure 3

Relation of PM2.5 concentration with air mass transport trajectories and residence time in areas around Northern Vietnam

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In REAS inventory data set (Kurokawa and Ohara 2020), national BC emissions in Vietnam was estimated as 59 Tg/y in 2015, second largest in ASEAN countries. Given the significant climate forcing of BC, this study strongly suggests that mitigation measures to reduce BC in Vietnam can considerably improve both regional climate change and air quality in the Northern Vietnam region.